James Genus was born in Hampton, Virginia on January 20, 1966. He started on guitar at six and moved to bass at thirteen, and his teenage apprenticeship was less a jazz education than a working one: talent shows around the Virginia peninsula with the Wooten brothers, all-night jam sessions, a long-running gig at Busch Gardens Williamsburg, and a stint in a country band, taken alongside every other job he could get. He studied at Virginia Commonwealth University from 1983 to 1987 and picked up the double bass in his sophomore year for the plainest of reasons: the degree required it. By his own account it was rough at first, since he was still playing electric constantly, but he ended up devoted to the instrument, and the doubling became the thing his whole career is built on.

He moved to New York after graduating and was working almost immediately. Between 1988 and 1992 he passed through the young-lions band OTB, Horace Silver's group, and bands led by Roy Haynes, Don Pullen, Nat Adderley, Greg Osby, Jon Faddis, T.S. Monk, Benny Golson, David Kikoski and Bob Berg. His first significant recorded appearance is Don Pullen's Random Thoughts, cut digitally at A&R Recording Studio in New York on March 23, 1990 for Blue Note in a trio with Lewis Nash. Within eighteen months he had recorded Nat Adderley's The Old Country at Clinton Studio on December 5 and 6, 1990, Jon Faddis's Hornucopia for Epic (where he splits the record between acoustic bass on five tracks and electric on four, and co-wrote "Dewey's Dance" with Faddis, Renee Rosnes and Billy Drummond), T.S. Monk's Take One at Van Gelder Studio on October 16, 1991, and the Benny Golson Quintet's Domingo, made in Paris on November 11, 12 and 13, 1991 with Curtis Fuller, Kevin Hays and Tony Reedus.

The association that widened his audience began with a phone call. Genus was on the road when Michael Brecker rang to say he was one of the bassists under consideration for the Brecker Brothers reunion, a band whose records Genus had grown up on; Don Grolnick's Hearts and Numbers was, he later said, his favourite Brecker recording of all. He plays acoustic and electric bass on five of the eleven tracks of Return of the Brecker Brothers (GRP, 1992) and on six of the nine on Out of the Loop (GRP, 1994), which won the Grammy for Best Contemporary Jazz Album at the 37th ceremony, sharing the bass chair on both with Armand Sabal-Lecco. He went on to make Michael Brecker's own Two Blocks from the Edge (Impulse!, 1998) in a quartet with Joey Calderazzo, Jeff "Tain" Watts and Don Alias, and he was in the Steps Ahead line-up Mike Mainieri assembled for Vibe in 1995.

His longest and most closely documented jazz relationship is with Dave Douglas. It runs from In Our Lifetime, recorded at the Power Station on December 7 and 8, 1994, through Stargazer (1996), Magic Triangle (1997), Moving Portrait (1997), Leap of Faith (1998) and Soul on Soul (1999), the last of them a Mary Lou Williams project on which Genus plays twelve of the thirteen pieces. From 2001 the relationship settled into a working quintet with Uri Caine on Fender Rhodes, Clarence Penn on drums, and Chris Potter, later Rick Margitza and Donny McCaslin, on tenor. That band made The Infinite at Avatar Studios on December 16, 17 and 18, 2001, Strange Liberation in January 2003 with Bill Frisell added on guitar, Live at the Bimhuis in Amsterdam, Meaning and Mystery at Bear Tracks on February 1, 2006, and Live at the Jazz Standard, taped across six nights in December 2006. Alongside it he worked steadily for Caine's own projects, playing on the trio record Blue Wail (1997), the street-recorded Sidewalks of New York: Tin Pan Alley (1999) and, as one of four bassists spread across a 72-piece set, The Goldberg Variations (2000).

A second standing group ran through the same years in Japan. Genus and Clarence Penn were the rhythm section of Makoto Ozone The Trio from Pandora (2000) to Dimensions (2017), by way of Treasure (2002, which added Chick Corea, Michael Brecker, Gary Burton and Jon Hendricks), Reborn (2003), New Spirit (2004), Real (recorded April 18 to 20, 2005), First Decade (2006) and the Blue Note Tokyo live set Alive!! (2007). That band is also the only place his writing appears with any regularity: he co-produced Real and wrote two of its ten pieces, "Central Booking" and "October Song," while "Caminos de Abrojos" and "Three the Hard Ways" are his as well. The same trio, with the teenage Julian Lage added on guitar, is the band on Gary Burton's Generations, recorded at Fantasy Studios in Berkeley in September 2003.

In 2000 he took the electric bass chair in the Saturday Night Live Band, first heard on the season premiere of October 7, 2000, and he has held it since, which makes him one of the most widely heard jazz bassists in the United States and one of the least often named. The television job sits beside a heavy commercial session book, and it did not displace the jazz work. He recorded three albums with Ravi Coltrane between 1999 and 2012, played on Alice Coltrane's comeback Translinear Light (2004) opposite Charlie Haden, and made James Carter's Present Tense (2008), Bob James and David Sanborn's acoustic quartet album Quartette Humaine (2013) with Steve Gadd, Dianne Reeves's Beautiful Life (2013) and Tia Fuller's Diamond Cut (2018), where he and Bill Stewart hold half the record and Dave Holland and Jack DeJohnette the other half. He began working in Herbie Hancock's band in 2008 and was still touring with it more than a decade later, photographed in the group in Warsaw in November 2010 on the Imagine Project tour and at the Zelt-Musik-Festival in Freiburg in July 2017. He also plays bass on six of the thirteen tracks of Daft Punk's Random Access Memories (2013), and has been the bassist of the New York art-rock band Elysian Fields since their first record in 1996.

The most recent chapter has brought him back to straight-ahead playing. He appeared on five Oz Noy records between 2003 and 2020, always as one of two or three rotating bassists, before becoming the sole bassist on Fun One, cut for Criss Cross Jazz at Samurai Hotel Recording Studio on July 26 and 27, 2024 in a quartet with David Kikoski and Clarence Penn. Around it sit Kenny Werner's Church on Mars, recorded for Newvelle on October 8, 2018 with Dave Liebman and Terri Lyne Carrington, Clarence Penn's Behind the Voice (Origin, 2024), and the Brooklyn band Forq's Big Party (2024). After close to four decades and several hundred sessions he has never made an album under his own name alone. Asked about it in 2009 he said he had been asked many times, did not want to do it just to do it, and wanted to be sure that whatever he put out said "this is me." Nothing has appeared since.

Genus is the rare bassist whose reputation rests equally on both instruments, and he describes the doubling as something that happened to him rather than something he planned: he took up the double bass because a degree required it, and when he arrived in New York a lot of people did not know he played electric at all. What he brings to either is a deep, unhurried, notably quiet authority. He has said the first thing he does in any situation is establish a connection with the drummer, that the push and pull between the two is where the feel actually lives, and that if that connection is not made nothing else will sound right, which is a fair description of what the Dave Douglas quintet records sound like from the bottom up. He is also unusually genre-agnostic for a player of his standing, having spent twenty-five years alternating between a Blue Note hard bop date, a network television house band, a Bach recomposition and an electronic pop record without any audible change of manner. His own formulation is that he tries to be literally the bass player and to react to the music in front of him rather than bring a style to it. On the acoustic instrument, as on Douglas's Meaning and Mystery, the tone is broad and woody and the time enormous; on the five-string electric he is a groove player of the school that Anthony Jackson and Marcus Miller made possible, felt more than heard. He writes rarely, but the four tunes he has contributed to Makoto Ozone's trio records suggest the reticence is a choice rather than a limit.